SQL & PL/SQL

Hi All,

I'm getting an invalid Identifier error on my web page when the below procedure is called.
I'm trying to Order my Query by the 'p_Group' input parameter passed into the procedure.
p_Group has a value of 1 or 2, i.e. I need to order by either the first (a.assign_To) or second (Status) Column returned by the Query.

Has anyone any Idea why this error is happening and how I can rectify it.

thanks in advance
C


PROCEDURE GetData
(p_Group IN NUMBER DEFAULT 1, po_DataSet OUT t_cursor)

IS

BEGIN

OPEN po_Dataset
FOR
SELECT a.assign_To, 'OPEN' AS Status, COUNT(a.assign_To) AS COUNT, p_Group
FROM DAT_TWH_HEADERS TH, DAT_TWH_DETAILS TD, ADM_ASSIGN_TO a
WHERE
TD.TWH_HEADER_ID (+)= TH.TWH_HEADER_ID
AND TH.Status = 'OPEN'
AND A.ASSIGN_TO_ID (+)= TH.ASSIGN_TO_ID
AND TH.Item_Type = 'HZ'
GROUP BY a.Assign_To

UNION

SELECT a.assign_To, 'CLOSED' AS Status, COUNT(a.assign_To) AS COUNT, p_Group
FROM DAT_TWH_HEADERS TH, DAT_TWH_DETAILS TD, ADM_ASSIGN_TO a
WHERE
TD.TWH_HEADER_ID (+)= TH.TWH_HEADER_ID
AND TH.Status = 'CLOSED'
AND A.ASSIGN_TO_ID (+)= TH.ASSIGN_TO_ID
AND TH.Item_Type = 'HZ'
GROUP BY a.Assign_To

UNION

SELECT a.assign_To, 'OVERDUE' AS Status,COUNT(a.assign_To) AS COUNT, p_Group
FROM DAT_TWH_HEADERS TH, DAT_TWH_DETAILS TD, ADM_ASSIGN_TO a
WHERE TD.TWH_HEADER_ID (+)= TH.TWH_HEADER_ID
AND A.ASSIGN_TO_ID (+)= TH.ASSIGN_TO_ID
AND TH.Item_Type = 'HZ'
AND TH.Status = 'OPEN'
AND TH.CLOSE_Date IS NULL
AND TH.Target_Completion_Date < SYSDATE
GROUP BY a.Assign_To
ORDER BY p_Group;
END;